The present invention relates to a method of determining a surface illuminated by incident light by recording the intensity of light reflected from the area in a first image thereof and recording the intensity of light reflected from the area in a second image thereof, complementary to the first image, taken with another angle of illumination.
The invention is particularly but not exclusively applicable to paper surfaces intended for the application of print.
SE 508 822 makes known a method and a device for measuring and quantifying surface defects, such as polishing roses that can occur in connection with the polishing of coated sheet metal items. In this method and device, at least two sub-images are recorded with at least one camera under illumination of the lest surface with parallel light or light from a point source, whereby the angles of incidence of the light relative to the test surface and/or the camera are different during the recording of different sub-images, after which the sub-images are processed in at least one central unit. After this, one or several difference images of the sub-images are produced, and used to determine the degree of surface defects on the test surface. This known technique, however, provides no guidance in how the recorded sub-images can be used in order to determine the topography of the surface.
An object of the present invention is to provide a photometric method of the type described in the introduction that can rapidly determine the topography of a surface. According to an aspect of the invention, the intensity (that is, the power per unit area) only of diffusely reflected light is recorded in the two images, and a difference between the recorded intensities of the diffusely reflected light of the first and the second recorded images is determined, in order to obtain a representation of the gradient variations of the surface.
If the difference is normalized by division by the sum of the intensities, a ratio is obtained that is essentially directly proportional to the local derivative of the surface.
The derivative in turn is used to determine the height function of the surface.
The insight that forms the basis of the invention is that the lightness of a topographic surface element depends both on its diffuse reflectance and on its angle relative to the illumination. If images of the surface are taken with different angles of illumination, these will differ due to the topography of the surface, but not due to differences in its diffuse reflectance. This can, according to the invention, be used in image processing operations that distinguish the topography from the reflectance.